Chemical mechanical polishing for forming a shallow trench isolation structure

ABSTRACT

A method of chemical-mechanical polishing for forming a shallow trench isolation is disclosed. A substrate having a number of active regions, including a number of relative large active regions and a number of relative small active regions, is provided. The method comprises the following steps. A silicon nitride layer on the substrate is first formed A number of shallow trenches are formed between the active regions An oxide layer is formed over the substrate, so that the shallow trenches are filled with the oxide layer. A partial reverse active mask is formed on the oxide layer. The partial rever active mask has an opening at a central part of each relative large active region. The opening exposes a portion of the oxide layer. The opening has at least a dummy pattern. The oxide layer on the central part of each large active region is removed to expose the silicon nitride layer. The partial reverse active mask is removed The oxide layer is planarized to expose the silicon nitride layer.

CROSS-REFERENCE TO RELATED APPLICATION

[0001] This application claims the priority benefit of Taiwanapplication serial no 87108699, filed Jun. 3, 1998, the full disclosureof which is incorporated herein by reference

BACKGROUND OF THE INVENTION

[0002] 1. Field of the Invention

[0003] This invention relates to a chemical mechanical polishing (CNP)applied in forming shallow trench isolation (STI), and moreparticularly, to a processs of forming a STI structure combining CMP,using a partial reverse active mask.

[0004] 2. Description of Related Art

[0005] CMP is now a technique ideal for appling in global planarizationin very large scale integration (VLSI) and even in ultra large scaleintegration (ULSI). Moreover, CMP is likely to be the only reliabletechnique as the feature size of the integrated circuit (IC) is highlyreduced. Therefore, it is of great interest to develope and improve theCMP technique in order to cut down the cost.

[0006] As the IC devices are contineously sized down to a linewith of0.25 μm or even 0.18 μm (deep sub-half micron), using CMP to planarizethe wafer surface, especially to planarize the oxide layer on thesurface of the shallow trench, becomes even more important. To preventthe dishing effect occuring at the surface of a larger trench during CMPprocess and to obtain a superior CMP uniformity, a reverse tone activemask was proposed, cooperated with an etching back process.

[0007] Typically, the activel regions have varied sizes and the shallowtrenches between the active regions also have different sizes FIG. 1A toFIG. 1E are cross sectional veiws showing the process steps for formingshallow trench isolation, using CMP Referring to FIG. 1A, on a substrate10, a pad oxide 15 and a silicon nitride layer 16 are depositedsuccessively. By photolithography, the substrate 10, the pad oxide layer15 and the silicon nitride layer 16 are anisotropically etched to formshallow trenches 14 a, 14 b, 14 c and define active regions 12 a, 12 b,12 c, 12 d. The sizes of the shallow trenches 14 a, 14 b, 14 c aredifferent since the sizes of the active regions 12 a, 12 b, 12 c, 12 dare varied.

[0008] Next, referring to FIG. 1B, an oxide layer 18 is deposited byatmosphere pressure chemical vapor deposition (APCVD) on a substrate 10to fill the interior of the shallow trenches 14 a, 14 b, 14 c. However,due to the step coverage of the oxide layer 18, the deposited oxidelayer 18 has an uneven surface and a rounded shaped. Then, a photoresistlayer is coated on the surface of the oxide layer 16 and patterned toform a reverse active mask 20 by photolithography. The reverse activemask 20 covers the shallow trenches 14 a, 14 b, 14 c and iscomplementary to the active regions 12 a, 12 b, 12 c, 12 d However,during the formation of the reverse active mask, misalignment causes theoxide layer 18 to cover more than the shallow trenches 14 a,14 b,14 c

[0009] Referring to FIG. 1C, the oxide layer 18 exposed outside thereverse active mask 20 is etched until the silicon nitride layer 16 isexposed so that only a part of the silicon oxide layer 18, the siliconoxide layer 18 a, is formed. After removing the reverse active mask 20,as shown in FIG. 1D, it is obserable that the silicon oxide layer 18 aremained does not fully cover the shallow trenches 14 a, 14 b, 14 c atone sides of the shallow trenches 14 a, 14 b, 14 c, therefore, formingcavities 22, but at the other sides over-cover the shallow trenches 14a, 14 b, 14 c, forming photo-overlape 24

[0010] Referring to FIG. 1E, the portion of the oxide layer 18 a higherthan the shallow trenches 14 a, 14 b, 14 c is polished by CMP until thesurface of the silicon nitride layer 16 is exposed. Therefore, thesilicon nitride layer 16 and the silicon oxide layer 18 a are at thesame level. The profile of the silicon oxide layer 18 a formed by APCVDis rather rounded and the APCVD silicon oxide layer 18 a is hard to beplanarized. Moreover, it is obvious that the silicon oxide layer 18 adoes not fully fill the shallow trenches 18 a, 18 b, 18 c but form theconcaves 22. The undesired concaves 22 may cause kink effect andconsequent short circuit or leakage current which therefore influencethe yield.

[0011] As a result, it is important to overcome the problems comingafter the formation of the concaves due to the misalignment of thereverse active mask during the process of CMP, especially, whilenowadays the linewidth is decreasing.

SUMMARY OF THE INVENTION

[0012] It is therefore an objective of the present invention to providea method of chemical-mechanical polishing for forming a shallow trenchisolation. A substrate having a number of active regions, including anumber of relative large active regions and a number of relative smallactive regions, is provided. The method comprises the following steps. Asilicon nitride layer on the substrate is first formed. A number ofshallow trenches are formed between the active regions. An oxide layeris formed over the substrate, so that the shallow trenches are filledwith the oxide layer. A partial reverse active mask is formed on theoxide layer. The partial rever active mask has an opening at a centralpart of each relative large active region The opening exposes a portionof the oxide layer. The opening has at least a dummy pattern The oxidelayer on the central part of each large active region is removed toexpose the silicon nitride layer The partial reverse active mask isremoved The oxide layer is planarized to expose the silicon nitridelayer.

BRIEF DESCRIPTION OF DRAWINGS

[0013] The invention can be more fully understood by reading thefollowing detailed description of the preferred embodiments, withreference made to the accompanying drawings, wherein

[0014]FIG. 1A to FIG. 1E are cross-sectional views showing the processsteps of forming a conventional shallow trench using a reverse activemask,

[0015]FIG. 2A to FIG. 2E are cross-sectional views showing the processsteps of forming shallow trenches using a partial reverse active maskaccording to a preferred embodiment of the invention, and

[0016]FIG. 3A to FIG. 3D illustrate the partial reverse active maskaccording to a preferred embodiment of the invention.

DETAILED DESCRIPTION OF PREFERRED EMBODIMENTS

[0017] The invention provides a process for forming STI, combining thepartial reverse active mask and CMP, using high density plasma chemicalvapor deposition (HDCVD). This process prevents the formation ofconcaves in the shallow trenches due to the misalignment of the reverseactive mask, which consequently causes short circuit or leakage current.

[0018] Referring to FIG. 2A, active regions 42 a, 42 b are defined on asubstrate 40 first by depositing a pad oxide layer 45 and a siliconnitride layer 46, and then by photolithography and trench etching toform shallow trenched 44 between the active regions 42 a, 42 b The sizesof the shallow trenshes are varied since the sizes of the active regions42 a, 42 b are different. Then, a silicon oxide layer 48 is depositedover the substrate 40 and filling the trenches 44, preferred by highdensity plasma chemical vapor deposition (HDPCVD) The profile of thesilicon oxide layer 48 on the active region 42 a, 42 b is at a higherlevel than that of the silicon oxide layer 48 on the shallow trenches 44since the shallow trenches is fallen in the substrate 40. The HDPCVDoxide layer 48 on the active region 42 a,42 b has a sharp profile, asshown in FIG. 2B, which is different from the conventional.

[0019] Referring to FIG. 2C, a photoresist layer is coated on the oxidelayer 48 and difened to form a partial reverse active mask 50 byphotolithography. The partial reverse active mask 50 has an opening 52at the central part of the larger active region 42 a. Since the opening50 exposes only the central part of the silicon oxide layer 48 at thelarger active region 42 a, the silicon oxide layer 46 over the shallowtrenches 44 will not be exposed even though misalignment occurs.

[0020] Referring to FIG. 2D, using the reverse active mask 50 as a mask,the exposed silicon oxide layer 48 at the larger active region 42 a isetched back until the silicon nitride layer 46 is exposed. The reverseactive mask is then peeled. Then, only the oxide layer 48 b on thesmaller active region 42 b and a small portion of the silicon oxidelayer 48 a through etching back on the larger active region 42 a areremained. The remained silicon oxide layer 48 a and 48 b formedpreferrably by HDPCVD have sharp profile and is therefore easy to beplanarized by CMP. Also, the sizes of the remained silicon oxide layer48 a and 48 b are more or less similar so that the consistence of CMP isincreased

[0021] Next, referring to FIG. 2E, the remained silicon oxide layer 48 aand 48 b (as shown in FIG. 2D) are polished by CMP, using the siliconnitride layer 46 as an etching stop layer so that the silicon oxidelayer 48 c in the shallow trenches and the silicon nitride layer 46 arealmost at the same level

[0022] In the above embodiment, a partial reverse active mask isemployed for forming a shallow trench isolation In FIG. 3A to FIG. 3D, amethod of forming a partial reverse active mask is shown. As shown inFIG. 3A, to define a photo-mask pattern, active regions are formedfirst. The active regions include a larger active region pattern 60 anda smaller active region pattern 62.

[0023] Referring to FIG. 3B, the larger active region pattern 60 and thesmaller active pattern region 62 are shrunk as shown in the figure. Theshrinking larger active region pattern and the shrinking smaller activeregion pattern are denoted as 60 a and 62 a respectively.

[0024] Refering to FIG. 3C, the shrinking process is continued until theshrinking smaller active region pattern 62 a disappears. The shrinkingdistance is about 0.5 μm to 2 μm each side so that active regionpatterns with maximun radius of less than 1˜4 μm will disappear. Next,the shrinking larger active region 60 a is enlarged until the profile ofit is a little bit smaller than the profile of the original largeractive region pattern. The profile of the larger active region patternat the stage is denoted as 60 b. The shrinking large active regionpattern 62 a is enlarged with a dimension of about 0.2 μm to 2 μm eachside. This enlarged dimension is smaller than the shrinking distancementioned above.

[0025] Referring to FIG. 3D, the partial reverse active mask 60 b islocated at the central part of the larger active region 60 but slightlysmaller than the larger active region One characteristic of the presentinvention is that the partial reverse active mask pattern 60 b at thelarger active region 60 has dummy pattern 64 so that dishing effect atthe larger active region 60 can be avoided. By applying this photo-maskpattern in forming a shallow trench isolation, the central part of anactive region is exposed, whereas the edge part of the active region iscovered by a photo-resist. A partial reverse active mask pattern is thusobtained

[0026] The advantages of the invention are.

[0027] (1) The oxide layer formed by HDCVD has a pyramid-like profile,so that using chemical-mechanical polishing, the oxide layer isplanarized easily.

[0028] (2) Using a partial reverse active mask to etch away the oxidelayer on the central part of an active region, only the oxide layer onthe edge part of the active region and on a small active region isremained. The profile of the remaining oxide layer is pyramid-like andhas a better uniformity Therefore, a recess formed while polishing alarge trench is avoided.

[0029] (3) The dishing effect on the large active region is avoidedsince the partial reverse active mask has a dummy pattern.

[0030] (4) Since only the oxide layer on the central part of an activeregion is etched away by using a partial reverse active mask, even whena misalignment occurs, the oxide layer within the trench is not etched.The kink effect is prevented. As a consequence, the current leakage andthe short circuit caused by kink effect are avoided, so that the yieldof wafer is enhanced.

[0031] Other embodiment of the invention will appear to those skilled inthe art from consideration of the specification and practice of theinvention disclosed herein It is intended that the specification andexamples to be considered as exemplary only, with a true scope andspirit of the invention being indicated by the following claims

What is claimed is:
 1. A method of chemical-mechanical polishing forforming a shallow trench isolation, wherein a substrate having aplurality of active regions, including a plurality of relative largeactive regions and a plurality of relative small active regions, isprovided, comprising. forming a silicon nitride layer on the substrate;forming a plurality of shallow trenches between the active regions,forming an oxide layer over the substrate, so that the shallow trenchesare filled therewith, forming a partial reverse active mask on the oxidelayer, wherein the partial rever active mask has an opening at a centralpart of each relative large active region, wherein the opening exposes aportion of the oxide layer, and wherein the opening has at least a dummypattern; removing the oxide layer on the central part of each largeactive region to expose the silicon nitride layer therewithin; removingthe partial reverse active mask; and planarizing the oxide layer toexpose the silicon nitride layer.
 2. A method as claimed in claim 1,wherein the shallow trenches are formed by photolithography and etching.3 A method as claimed in claim 1, wherein the oxide layer is formed byhigh density plasma chemical vapor deposition. 4 A method as claimed inclaim 1, wherein the exposed portion of the oxide layer is removed byanisotropic etching 5 A method as claimed in claim 4, wherein theexposed portion of the oxide layer is removed, using the silicon nitridelayer as an etching stop layer. 6 A method as claimed in claim 1,wherein the oxide layer is planarized by chemical mechanical polishing.7. A method of forming a partial reverse active mask pattern, applied infabricating shallow trench isolation, wherein the method comprises.providing a mask pattern, wherein the mask pattern comprises a pluralityof relative large active region patterns and a plurality of relativesmall active region patterns; shrinking the relative large active regionpatterns and the relative small active region patterns until therelative small active region patterns disappear and the relative largeactive region patterns become a remaining relative large active regionpatterns; and enlarging the remaining relative large active regionpatterns so that the remaining relative large active region patterns aresubstantially smaller than original profiles of the relative largeactive regions and each of the relative large active region patterns hasat least one dummy pattern. 8 A method as claimed in claim 7, wherein insaid step of shrinking the relative large active region patterns and therelative small active patterns, a shrinking size is about between 0.5 μmand 2 μm. 9 A method as claimed in claim 7, wherein in said step ofenlarging the remaining relative large active region patterns, anenlarging size is about between 0.2 μm and 2 μm 10 A method as claimedin claim 7, wherein an enlarging size in said step of enlarging theremaining relative large active region patterns is substantially smallerthan a shrinking size in said step of shrinking the relative largeactive region patterns and the relative small active patterns.